Electrical inductive apparatus

ABSTRACT

An electrical coil of the flat disc or pancake type for electrical inductive apparatus, such as power transformers. The coil has a plurality of conductor turns formed of a stranded conductor assembly having a length dimension L. The conductive strands of the conductor assembly are arranged into two adjacent sections, each having a plurality of adjacent layers of one or more strands. The strands of the conductor assembly are transposed with a complete transposition at substantially 1/4 L and at substantially 3/4 L, directing each layer of each section to a layer position in the other section which is in 180* rotational symmetry with its previous layer position. The strands of the conductor assembly are transposed at substantially 1/2 L with a standard transposition of the layers, directing each layer of each section to the same layer position of the other section.

United States Patent lnventors Harold R. Moore Muncie; Virgil L. Benz, Dalevllle; William B. Wallace, Muncie, all of, lnd.

Appl. No. 876,769

Filed Nov. 14,1969

Patented Aug. 31, 197i Assignee Westinghouse Electric Corp.

Pittsburgh, Pa.

ELECTRICAL INDUCTIVE APPARATUS 12 Claims, 5 Drawing Figs.

us. m 336/187 Int. Cl. 1101f 27/28 Field of Search. 336/69, 70, 180, 186, 187

References Cited UNITED STATES PATENTS 1,629,462 5/1927 Paluefi' 336/187 2,710,380 6/1955 DeBuda 3,829,355 4/1958 Eberle ABSTRACT: An electrical coil of the flat disc or pancake type for electrical inductive apparatus, such as power transformers. The coil has a plurality of conductor turns formed of a stranded conductor assembly having a length dimension L. The conductive strands of the conductor assembly are arranged into two adjacent sections, each having a plurality of adjacent layers of one or more strands. The strands of the conductor assembly are transposed with a complete transposition at substantially /4 L and at substantially L, directing each layer of each section to a layer position in the other section which is in 180 rotational symmetry with its previous layer position. The strands of the conductor assembly are transposed at substantially L with a standard transposition of the layers, directing each layer of each section to the same layer position of the other section.

PATENTED AUBBI lsm SHEET 2 OF 3 PATENTEU AUGBI I971 SHEET 3 BF 3 ELECTRICAL I NDUCTIVE APPARATUS BACKGROUND OF THE INVENTION A 1. Field of the Invention The invention relates in general to electrical inductive apparatus, such as power transformers, and more specifically to pancake-type coils for such apparatus which have a plurality of conductor turns formed of a stranded conductor assembly.

2; Description of the Prior Art It is conventional in prior art pancake coils for electrical inductive apparatus, such as power transformers of the shellform type, to wind the coils with a stranded conductor assembly. The volume of the conductive material required is subdivided into insulated, parallel connected strands. Subdividing the required conductive material reduces losses due to eddy currents produced by the leakage flux, as eddy current .losses are approximately proportional to the square of the conductor dimension at right angles to the direction of the leakage flux components. To prevent an offsetting increase in losses due to circulating currents in the parallel connected strands, the relative positions of the strands are transposed with respect to the leakage flux, in an attempt to obtain the same net flux linkages for each strand. In double section con ductor assemblies, wherein each section has a plurality of layers of one or more conductive strands,, it has been the practice in untapped coils to transpose the layers from one section to the other, maintaining the same layer position, once during the winding of the coil, which is called a standard transposition of the layers. Between taps on a tapped coil, a socalled complete transposition has been used, which directs each layer of each section to the other layer, to a layer position which is in 180 rotational symmetry with its previous layer position. While these prior art transposing arrangements have been satisfactory in the past, the trend towards increased voltage and power ratings of electrical inductive apparatus requires even larger cross-sectional areas for conductor as semblies used in the coils, requiring more conductive material, more conductive strands, and additional cooling apparatus to remove the heat due to increased losses.

Thus, it would be desirable to provide a new and improved electrical coils of the flat, pancake type, having conductor turns formed of a stranded conductor assembly, which has lower. losses dueto circulating currents than similarly rated coils of the prior art.

SUMMARY OF THE INVENTION Briefly, the present invention is a new and improved coil of the pancake type for electrical inductive apparatus, with the coil having a plurality of spirally wound conductor turns formed of a standard conductor having a length dimension L. The standard conductor has first and second sections, with each section having a plurality of adjacent layers of one or more conductive strands. The conductor assembly has its conductive strands transposed with complete transpositions at .substantially L and 54 L, while the layers of the two sections are transposed with standard transpositions at substantially /2 L. These transpositions are located as close to these locations as allowed by the physical characteristics of the transformer, as transpositions can only be placed in the top or bottom portions of the coil. This critical placement of complete and standard transpositions in a pancake coil provides a significant reduction in circulating current, making it unnecessary to tions at substantially the one-fourth and three-fourths points of the conductor length, taking advantage of the pattern of both the axial and radial components of the leakage field.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description and drawings in which:

FIG. 1 is an elevational view, in section, of electrical inductive apparatus of the type which may advantageously use coils constructed according to the teachings of the invention;

FIG. 2 is a diagrammatic representation of a conductor suitable for winding a double section pancake coil, of the type having a single conductive strand per layer per section, with the strands being transposed accordingto an embodiment of the invention;

FIG. 2A is a diagrammatic representation which illustrates a modification of the embodiment shown in FIG. 1;

FIG. 3 is a diagrammatic representation of a conductor as sembly suitable for winding double section pancake coils, with the conductor being of the type having two conductive strands per layer per section, constructed according to another embodiment of the invention; and

FIG. 4 is a diagrammatic representation of a conductor assembly suitable for winding a double section pancake coil of the type having two conductive strands per layer per section, constructed according to still another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and FIG. I in particular, there is shown electrical inductive apparatus 10 of the shellform type, such as a power transformer or reactor, having a magnetic core-winding assembly 12 disposed in a tank 14 filled to a predetermined level 16 with a fluid cooling and insulating medium, such as mineral oil. The magnetic core-winding assembly 12 includes a plurality of flat, disc or pancaketype coils, such as pancake coil 18, which coils are stacked with their openings in alignment. In other words, the coils of the highand low-voltage windings of an electrical phase are axially spaced, rather than being concentrically disposed, and the coils of the highand low-voltage windings may beinterleaved axially, if desired. Each coil, such as coil 18, has'a plurality of conductor turns 19 formed of a strandedconductor assembly having first and second ends 21 and 23, respectively. The coils are disposed in inductive relation with magnetic core sections 20 and 22, which sections are formed of a plurality of stacked magnetic laminations each having an opening or window per phase. The magnetic core sections 20 and 22 are disposed in adjacent relation with the coils encircling the common leg portion formed by the adjacent core sections.

Since the dimensions of the coils, such as coil 18, are necessarily fixed in the portion of the coil which extends through the openings in the magnetic core sections, transposition must be performed in the portions of the coil where increases in the radial dimension of the coil can be tolerated, such as in the space 24 at the bottom portion of the coil, and also, in untapped coils, in the space 26 at the top portion of the coil.

In electrical inductive apparatus of the shell-form type, the leakage flux components are perpendicular to the axis of the coils, or in the plane of the coils. The leakage field, however, does not link uniformly with the conductor turns of a coil across its build dimension, having substantial vertical and horizontal components adjacent the inner and outer edges of the coil, while having mainly a vertical component nearthe middle portion of the coil build. Thus, the prior art arrangements, while being satisfactory at lower voltages and power,

allow substantial circulating currents and losses at the extra' high-voltages and power ratings of the present day apparatus.

More specifically, the invention involves the critical placement of complete and standard transpositions within a single pancake coil. The pancake coil includes a plurality of conductor turns formed of a stranded conductor assembly having a length dimension L. The conductor assembly has first and second adjacent sections, each of which have a plurality of adjacent layers of one or more electrically conductive strands. The conductor assembly has its strands transposed with complete transpositions at substantially A L and A L, and with standard transpositions of the layers at substantially L. If the /4 L, A L or 36, L points fall within the iron of the core, the transpositions are made at the closest end position, i.e., either within space 24 or within space 26. Thus, the complete transpositions will be located within the indicated spaces on lines 30 and 32 of pancake coil 18, and the standard transpositions will be located within the indicated spaces on line 34.

FIG. 2 is a diagrammatic representation of a conductor assembly 40 having first and second ends 42 and 43, respectively, a length L, a first transposition 44 at substantially A. L, a

second transposition 46 at substantially /2 L, and a third trans-.

position 48 at substantially L. Cross sections of conductor assembly .40 are shown at these points to illustrate the positions of the conductive strands at the start and finish of the coil, as well as immediately before, during, and at the completion of each transposition.

Conductor 40 has first and second adjacent sections I and II, respectively, each having a plurality of layers, with five layers being used in this example, and with each layer of each section having a single conductive strand. Thus, as shown at the first end 42 of conductor assembly 40, section I has layers 50, 52, 54, 56 and 58, each having a single insulated conductive strand such as conductive strand 60 in layer 50, insulated with insulating means 61, such as enamel, or cellulosic insulation. The strands oflayers 50, 52, 54, 56 and 58 are numbered 1, 2, 3, 4 and 5, respectively. Section II has layers 50, 52', 54', 56' and 58', each having a single insulated conductive strand per layer, with the strands being numbered 10, 9, 8, 7 and 6 respectively. Adjacent layers of the two sections are given the same reference numeral, with a prime mark being used on the layers in section II, in order to denote that the layers are at the same level and adjacent one another.

Conductor 40 starts at its end 42, which end may be a startstart or finish-finish connection from an adjacent pancake coil. Sections 1 and II of conductor 40 are shown divided or spaced apart, in order to reduce circulating current in the interpancake connection, due to a vertical component of the leakage flux. 1

Conductor 40 is wound into a pancake coil having a plurality of radially disposed turns, with the strands of conductor assembly 40 being transposed with the first transposition 44 at dimension A L. This transposition is performed in the closest appropriate space to the A L. dimension, i.e., either in the lower space 24 or in the upper space 26, as illustrated in FIG. I. The first transposition 44 is a complete transposition, wherein each layer of each section is transposed to the opposite section, with the new layer position being in 180 rotational symmetry with the old position. The complete transposition is performed by bending the strands of the two sections in the plane of their respective sections, but in opposite directions in the two sections. As each strand reaches the outermost layer in the direction of the bends in that section, the layer is bent into the plane of the other section, and then bent to follow the bend direction of the new section, until the layer reaches its new layer position. FIG. 2 illustrates conductor 40, just prior to transposition 44 in cross-sectional view 62, during the steps of the transposition in cross-sectional views 64, 66, 68 and 70, and immediately following the transposition in view 72. The strands are bent upwardly in section I, and downwardly in section II, in this example, but their directions may be reversed, if desired. The first step, shown in cross-sectional view 64, bends the uppermost layer of section I into the plane of section II, bends the lowermost layer of section 11 into the plane of section I, bends the layers of section I upwardly to fill the vacated layer and to make room for the newly acquired layer from section II, and bends the layers of section II downwardly to fill the vacated layer, and to make room for the newly acquired layer from section l. This same procedure is followed, setting up a clockwise rotation of the layers, in this example, with each layer advancing to the other section, and stopping when it reaches its new layer position of that section. It will be noted that if an axis were to be inserted between the third and eighth strands of sections I and II, respectively, and the conductor rotated 180 about this axis, that the layers would be in the positions shown in the cross-sectional view 72 of the conductor 40.

After the completion of the first transposition 44, the pancake coil is wound until reaching the L dimension of the conductor, at which time the second transposition 46 is performed. The second transposition 46 is a standard transposition of each of the complete layers, effective for vertical components of the leakage flux. Since the standard transposition is performed by twisting the complete layer 180 about its longitudinal axis, it is preferable to stagger the standard transpositions of the layers, as illustrated in FIG. 2, in order to create as small a bulge in the radial build of the coil as possible. Crosssectional view 74 illustrates conductor 40 just prior to transposition 46, cross-sectional view 84 illustrates the conductor immediately following transposition 46, and cross-sectional views 76, 78, and 82 illustrate the various steps of the trans position. The first step of transposition 46, shown in cross-sectional view 76, exchanges the positions of layers 50 and 50', and the next step, shown in view 78, exchanges the positions of layers 52 and 52', the next step, shown in view 80 exchanges the positions of layers 54 and 54, the next step, shown in view 82, exchanges the positions of layers 56 and 56', and the last step, shown in view 84, exchanges the positions of layers 58 and 58'. I

Following the second transposition 46, the coil is wound until reaching the L dimension of the conductor, at which point the third transposition 48 is performed. The third transposition 48, like transposition 44, is a complete transposition, which is effective for both the horizontal and vertical components of the leakage flux, but, as illustrated in FIG. 2, it is preferable to perform the third transposition with a counterclockwise rotation of the strands, rather than the clockwise rotation of the first transposition 44, which eliminates twisting of the various strands from their supply reels. However, as shown in FIG. 2A, the third transposition 48, which is given the reference numeral 48' in FIG. 2A, may be performed in the identical manner of the first transposition 44. The relative positions of the strands at the end 43 of the coil are identical, whether the embodiment of FIG. 2 is followed for the third transposition, or the embodiment of FIG. 2A.

More specifically, in the third transposition 48 shown in FIG. 2, views 86 and 96 illustrate the relative positions of the strands prior to, and immediately following the transposition, respectively, with the views 88, 90, 92 and 94 illustrating the intermediate steps.

End 43 of the pancake coil may be a finish-finish connection between this pancake coil and the next adjacent pancake coil, or a start-start connection, and the two sections may be divided or spaced while interconnecting adjacent coils, similar to the spacing of the sections at end 42. I

The teachings of the invention may be applied with equal facility when each layer of each section has a plurality of conductive strands, such as two, with this embodiment of the invention being illustrated in FIG. 3. FIG. 3 is a diagrammatic representation of a conductor assembly having first and second ends 102 and 104, respectively, and a length dimension L. Conductor assembly 100 has first and second sections 1 and II with each section having five layers, in this embodiment, and with each layer having two conductive strands. For example, as illustrated at end 102 of conductor 100, section I has layers 112, 114, 116, 118 and 120, with the strands of each layer being given a number to identify the layer, and a letter to identify the strand of the layer. Thus, layer 1 12 of section I has strands 1A and 1B referenced 122 and 124, each insulated from one another and from the adjacent strands by insulating means 126. Section II of conductor 100 also has five layers, with the layers of section II being given the same reference numerals as like-positioned layers of section I, with the addition of a prime mark.

Conductor I00 starts at end 102, which may be a start-start or finish-finish interconnection between adjacent coils, and its two sections may be spaced to reduce circulating currents due to the vertical component of the leakage flux. Conductor 100 is wound into a pancake coil, with the first transposition 106 being disposed at the A L dimension of the conductor. The first transposition 106 is a complete transposition, with views 128 and 138 illustrating the relative positions of the strands before and after the transposition, respectively, and views 130, 132, 134 and 136 illustrating the various steps of the transposition. In this embodiment of the invention, the complete transposition 106 transposes the layers into the other section, in a layer position which is in 180 rotational symmetry with its previous position, but it should be noted that in this embodiment the individual strands of a layer are not in 180 rotational symmetry with their previous location, as the strands of a layer maintain their same relative position throughout the transposition.

Following the first transposition 106, the coil is wound until reaching the V2 L dimension of conductor 100, at which location the second transposition 108 is performed, by progressively twisting each complete layer 180 about its longitudinal axis. Views 140 and 150 illustrate the relative positions of the strands immediately prior to, and following the transposition 108, respectively, with views 142, 144, 146 and 148 illustrating the various steps of the transposing arrangement.

Following the second transposition 108, the pancake coil is wound until reaching the V4 L dimension of conductor 100, at which point the third transposition 110 is performed. Views 152 and 162 illustrate the relative positions of the conductive strands immediately prior to, and following, respectively, transposition 110. Views 154, 156, 158 and 160 illustrate the intermediate steps of transposition 110. It will be noted that the progression of the layers is counterclockwise in transposition 110, which is opposite to the rotation of the layers in the first transposition 106, in order to prevent twisting the various strands from their supply reels. It is to be understood, however, that the third transposition 110 may be performed in the identical manner of the first transposition 106, with the layers both progressing in the same circumferential direction.

Following the'third transposition 110, the pancake coil is wound until coming to the end 104 of conductor 100, with end 104 being a finish-finish or start-start connection, if desired, for interconnecting this coil with the next adjacent pancake coil. As illustrated, end 104 may have its sections I and II spaced or divided. if used as an interpancake connection. In the arrangement shown in FIG. 3, the first and third transpositions direct each layer of each section into the opposite section, to a layer location which is in 180 rotational symmetry with its previous layer position, but the strands of a layer maintain their same relative positions throughout the transpositions. Circulating currents may be reduced still further if the relative positions of the strands within the layer are transposed, using a standard transposition, which will result in the first and third transpositions providing true 180 rotational symmetry for the strands, prior to and following both the first and the third transpositions. This embodiment of the invention is illustrated in FIG. 4.

FIG 4 illustrates a conductor assembly 170 having first and second sections I and II, respectively, each of which have five layers, in this example, referenced 182, 184, 186, 188 and 190 in section I. The same reference numerals, except for a prime mark, are used for like-positioned layers of section II. Conductor 170 has first and second ends 172 and 174, respectively, and first, second and third transpositions 176, 178 and 180, respectively, which are located at substantially the A, V: and A L dimensions of the total length L of the conductor 170. End

172 of conductor may be a start-start or finish-finish connection, which interconnects adjacent pancake coils, with the two sections being spaced, if desired. Conductor 170 is used to wind the pancake coil, until reaching the A L dimension of the conductor, at which point the first transposition 176 is performed. In addition to performing a complete transposition, wherein layers of each section progress to the opposite section, to a layer position which is in 180 rotational symmetry with its previous layer location, the strands of each layer are twisted 180 about their longitudinal axis, to provide a stan dard transposition while the complete transposition is being performed. US. Pat. No. 3,283,280, which is assigned to the same assignee as the present application, teaches an arrangement for performing a standard transposition of the strands of a layer, while a complete transposition is being performed, or, as illustrated in FIG. 4, the standard transposition of the layers may be performed alternately at the inner and outer edges of the conductors. Views 192 and 202 illustrate the conductive strands immediately prior to, and following the transposition 176, respectively, with views 194, 196, 198 and 200 illustrating intermediate steps. Thus, as shown in view 194, the strands of layers 182 and 184 may be twisted to provide standard transposition of these layers at the upper edge of the conductor. As the complete transposition progresses, illustrated in view 196, the strands of layers 188' and 186' may be twisted at the bottom or lower edge of the conductor, to provide standard transpositions of these layers. The next standard transposition of the layers is performed at the upper edge of the conductor, as illustrated in view 198, providing standard transpositions of layers 188 and 186. The next standard transpositions are formed at the lower edge of the conductor, as illustrated in view 200, which provides standard transposition of layers 184' and 182'. The final standard transpositions of the layers are performed, as illustrated in view 202, by twisting the strands from layers 190' and 190.

After completing transposition 176, which provides a transposition wherein the layers and the strands of the layer'assume a position which is in 180 rotational symmetry with their previous position, the pancake coil is wound until reaching the A L dimension point of conductor 170, at which point transposition 178 is performed. Transposition 178 is a standard transposition of each of the layers, with views 204 and 214 illustrating the relative positions of the strands immediately prior to, and following the transposition 178, respectively. Views 206, 208, 210 and 212 illustrate the intermediate steps of this transposition.

Following transposition 178, the coil is wound until reaching the A L dimension of conductor 170, at which point transposition 180 is performed. Transposition 180, like transposition 176, is a complete transposition of the layers, and a standard transposition of the strands within a layer. Transposition 180 is similar to transposition 176, but instead of using a clockwise rotation of the layers throughout the transposition, a counterclockwise rotation is used to keep from twisting the strands from their supply reels. Also, in order to provide the same length for each strand between transpositions 176 and 180, the twisting of the strands ofa layer to provide standard transposition should follow the same sequence as the first transposition. Therefore, since strands 2A and 2B and strands 1A and 18 were transposed first in transposition 176, the same strands should be transposed first in transposition 180, which is illustrated in view 218. These strands appear at the bottom of the conductor 170, in view 218. The standard transposition of the next layers, therefore, are made at the top or upper edge of the conductor, as illustrated in view 220, then at the bottom as illustrated in view 222, and then at the top as illustrated in view 224. After completing transposition 180, the pancake coil is wound to completion, with end 174 serving as a finishfinish or start-start connectionbetween this coil and the next pancake coil. Sections I and Il may be divided, as illustrated in FIG. 4, if end 174 is used as an interpancake connection.

In summary, there has been disclosed a new and improved electrical coil of the pancake type for shell-form electrical inductive apparatus, such as power transformers, which has lower losses due to circulating currents than similarly rated coils of the prior art. A strategic placement within the coil of specific types of transposing arrangement, substantially reduces circulating currents between the parallel connected strands. By specifically tailoring the transposing arrangements to the leakage filed pattern at different portions of a pancake coil, a low-loss coil is obtained without resorting to the more costly continuously transposed type of coil.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings, shall be interpreted as illustrative, and not in a limiting sense.

We claim as our invention:

1. Apancake-type coil for electrical inductive apparatus comprising,

a plurality of conductor turns formed of a conductor assembly having first and second ends and a length dimension L,

said conductor assembly including first and second adjacent sections, each having a like-numbered plurality of adjacent layers,

each layer of each section having at least one electrically conductive strand,

said conductor assembly having its electrically conductive strands transposed with first, second and third transpositions located at substantially A L, 1% L and 541 L, respectively,

said first and third transpositions directing each layer of each section to the layer position of the other section which is in 180 rotational symmetry with its previous layer position,

said second transposition directing each layer of each section into the same layer position of the other section.

2. The pancake coil of claim 1 wherein the first and third transpositions are performed by bending the layers of the first and second sections in the planes of their respective sections, in opposite directions. with each layer being bent into the plane of the other section when it reaches the outermost layer of its original section.

3. The pancake coil of claim 2 wherein the directions of the bent layers in the first and second sections in the first transposition, are opposite to the directions of the bent layers of the first and second sections, respectively, in the third transpositron.

4. The pancake coil of claim 1, wherein the first and second sections are spaced apart by a predetermined dimension at the first and second ends of the conductor assembly.

5. The pancake coil of claim 1 wherein the second transposition is perfonned by twisting like layers of the first and second sections 180 about a longitudinal axis separating the sections.

6. The pancake coil of claim 1 wherein each layer of each section includes at least two electrically conductive strands disposed in side-by-side relationship.

7. The pancake coil of claim 6 wherein the strands of each layer maintain their positions relative to one another throughout the first transposition, and throughout the third transposition.

8. The pancake coil of claim 6 wherein each layer is twisted 180 about its longitudinal axis during the first transposition and during the third transposition, to provide 180 rotational symmetry for the strands of the layers, in addition to the 180 rotational symmetry of layer positions.

9. The pancake coil of claim 6 wherein the first and third transpositions are performed by bending the layers of the first and second sections in the planes of their respective sections, in opposite directions, with each layer being bent into the plane of the other layer when it reaches the outermost layer of its section.

10. The pancake coil of claim 9 wherein the directions of the bent layers in the first and second sections, in the first transposition, are opposite to the directions of the bent layers of the first and second sections, respectively, in the third transposition.

ll. The pancake coil of claim 6 wherein the first and second sections are spaced apart by a predetermined dimension at the first and second ends of the conductor assembly.

12. The pancake coil of claim 6 wherein the second transposition is performed by twisting like layers of the first and second sections l about a longitudinal axis separating the sections. 

1. A pancake-type coil for electrical inductive apparatus comprising, a plurality of conductor turns formed of a conductor assembly having first and second ends and a length dimension L, said conductor assembly including first and second adjacent sections, each having a like-numbered plurality of adjacent layers, each layer of each section having at least one electrically conductive strand, said conductor assembly having its electrically conductive strands transposed with first, second and third transpositions located at substantially 1/4 L, 1/2 L and 3/4 L, respectively, said first and third transpositions directing each layer of each section to the layer position of the other section which is in 180* rotational symmetry with its previous layer position, said second transposition directing each layer of each section into the same layer position of the other section.
 2. The pancake coil of claim 1 wherein the first and third transpositions are performed by bending the layers of the first and second sections in the planes of their respective sections, in opposite directions, with each layer being bent into the plane of the other section when it reaches the outermost layer of its original section.
 3. The pancake coil of claim 2 wherein the directions of the bent layers in the first and second sections in the first transposition, are opposite to the directions of the bent layers of the first and second sections, respectively, in the third transposition.
 4. The pancake coil of claim 1, wherein the first and second sections are spaced apart by a predetermined dimension at the first and second ends of the conductor assembly.
 5. The pancake coil of claim 1 wherein the second transposition is performed by twisting like layers of the first and second sections 180* about a longitudinal axis separating the sections.
 6. The pancake coil of claim 1 wherein each layer of each section includes at least two electrically conductive strands disposed in side-by-side relationship.
 7. The pancake coil of claim 6 wherein the strands of each layer maintain their positions relative to one another throughout the first transposition, and throughout the third transposition.
 8. The pancake coil of claim 6 wherein each layer is twisted 180* about its longitudinal axis during the first transposition and during the third transposition, to provide 180* rotational symmetry for the strands of the layers, in addition to the 180* rotational symmetry of layer positions.
 9. The pancake coil of claim 6 wherein the first and third transpositions are performed by bending the layers of the first and second sections in the planes of their respective sections, in opposite directions, with each layer being bent into the plane of the other layer when it reaches the outermost layer of its section.
 10. The pancake coil of claim 9 wherein the directions of the bent layers in the first and second sections, in the first transposition, are opposite to the directions of the bent layers of the first and second sections, respectively, in the third transposition.
 11. The pancake coil of claim 6 wherein the first and second sections are spaced apart by a predetermined dimension at the first and second ends of the conductor assembly.
 12. The pancake coil of claim 6 wherein the secOnd transposition is performed by twisting like layers of the first and second sections 180* about a longitudinal axis separating the sections. 